1. Field of the Invention
The present invention relates to a comparator circuit for discriminating input voltages and giving out a logic output indicating the discrimination results, and an infrared signal receiving apparatus that can be realized with such a comparator circuit.
2. Description of the Related Art
Conventionally, a comparator circuit 1 as shown in FIG. 8 has been used, for example, to discriminate integrated output voltages with a receiver of an infrared remote control apparatus or to discriminate outputs of a highly sensitive sensor or the like. The comparator circuit 1 employs a hysteresis comparator circuit 2 that has the hysteresis characteristics at the discrimination level. The hysteresis comparator circuit 2 compares an input voltage with the discrimination level, and when the input voltage becomes higher than the discrimination level, an output Vout is turned on. At the same time, the discrimination level is dropped so as to prevent the output Vout from being turned off, even if the input voltage is slightly varied and dropped. When the input voltage is further dropped to be lower than the dropped discrimination level, the output Vout of the hysteresis comparator circuit 2 is turned off, and the discrimination level is increased. Thus, the hysteresis threshold voltage Vth, which is the discrimination level of the hysteresis comparator circuit 2, is changed with the operational state.
Since in the hysteresis comparator circuit 2, the discrimination level has the hysteresis characteristics, malfunction that might occur inherently in circuits can be prevented, such as chattering in which an output is fluctuated between ON and OFF because the input voltage is fluctuated in the vicinity of the discrimination level when the discrimination level is fixed. In an infrared remote control receiver, for example, a photodiode which is an infrared receiving device receives an infrared signal that is modulated in an ASK (Amplitude Shift Keying) system, and a carrier frequency component is retrieved and detected, the hysteresis comparator circuit 2 determines whether or not a carrier is present while the detected output is integrated in an integrating circuit, and a digital output indicating determination results is given out. The determination results as to whether or not a carrier is present are processed in a logic circuit and thus are converted to digital signals. Since the input voltage for determining the presence of a carrier is an output from the integrating circuit, small fluctuations can readily occur because noise or the like superimposes, although a change is comparatively slow. It is possible to determine the presence of a carrier stably by using the hysteresis comparator circuit 2.
Conventionally, for power voltage for infrared remote control receiver, highly sensitive sensor circuits and the like, 5 V, which is a general power voltage for digital circuits, has been mainly used. In recent years, low power consumption and use of low voltage have been promoted for large-scale semiconductor integrated circuits (LSIs), so that there is a strong demand for use of a low power voltage of 3 V or less for infrared remote control receivers or highly sensitive sensor circuits as well. In particular, in a system employing batteries, there is a demand for ensuring operation at 2.4 V or 1.8 V as the lowest operation voltage. Although the hysteresis comparator circuit 2 serves to prevent malfunction that might occur inherently in circuits such as chattering, in order to permit stable operation at a low voltage, a sufficient hysteresis voltage width with respect to the discrimination level and a limiting circuit 3 for preventing saturation at an input portion are necessary. In addition, in order to reduce costs, it is necessary to configure a circuit having a simple and small circuit configuration that can be realized easily as a semiconductor integrated circuit.
The limiting circuit 3 limits an input voltage Vsig so as not to exceed a limit voltage Vlimit. When an integrating capacitor 5 with a capacitance C is charged with a signal current 4 denoted by Isig, the charging voltage constitutes the input voltage Vsig. The sum of a bias voltage supplied from a bias circuit 7 to the base of a PNP type transistor 6 of the limiting circuit 3 and a pn junction forward voltage between the base and the emitter of the transistor 6 constitutes the limit voltage Vlimit. When the input voltage Vsig exceeds the limit voltage Vlimit, conductivity occurs between the emitter and the collector of the transistor 6, and the impedance becomes low, so that the signal current 4 for charging the integrating capacitor 5 is absorbed to suppress the input voltage Vsig from increasing. The input voltage Vsig that is limited by the limiting circuit 3 is input to a hysteresis comparator 8 in the hysteresis comparator circuit 2 and is compared with a hysteresis threshold voltage Vth.
FIG. 9 shows signal processing waveforms in a principal portion of the comparator circuit 1 of FIG. 8. As shown by the solid lines, even if the input voltage Vsig is changed with variations, when the input voltage exceeds the hysteresis threshold voltage Vth, the output voltage Vout is changed from the high level to the low level. Here, the hysteresis comparator 8 operates in a negative logic in which when an output is off, the level is high, and when it is on, the level is low. With a transition of an output from OFF to ON, the hysteresis threshold voltage Vth is dropped by a hysteresis voltage width Vhis, so that even if the input voltage Vsig is fluctuated, chattering in which small fluctuations of an output occurs.
When the input voltage Vsig is increased and reaches the limit voltage Vlimit, the input voltage is limited by the limiting circuit 3 and does not exceed the limit voltage Vlimit. When the input voltage Vsig is started to drop and reaches the hysteresis threshold voltage Vth that is dropped from the original discrimination level by the hysteresis voltage width Vhis or becomes lower than that, then the output voltage Vout transits from the low level of being an ON state to the high level of being in an OFF state, and the hysteresis threshold voltage Vth is increased by the hysteresis voltage width Vhis. As the output voltage Vout, a pulse output corresponding to an overall change in which small fluctuations of the input voltage Vsig are ignored can be obtained. However, the pulse width in the ON state becomes longer by a period during which the input voltage Vsig is dropped from the limit voltage Vlimit to the hysteresis threshold voltage Vth that has been dropped. When the limiting circuit 3 is not provided, as shown in the broken line, the input voltage Vsig is increased further than the level shown by the limit voltage Vlimit, so that a period of time required for the input voltage to be dropped again becomes longer so that the pulse width is further increased.
A conventional technique related to a comparator with an integrator used for an infrared remote control receiver is disclosed in Japanese Unexamined Patent Publication JP-A 10-187862 (1998) by the present applicant. This conventional technique aims at reducing an influence of the temperature characteristics of a semiconductor on a reference voltage, which serves as the discrimination level, when a comparator circuit with an integrator is realized as a semiconductor integrated circuit.
The comparator circuit 1 shown in FIG. 8 is configured on the assumption that it is implemented in a semiconductor integrated circuit (IC). In semiconductor integrated circuits, various reference voltages such as the hysteresis threshold voltage Vth, the hysteresis voltage width Vhis, the limit voltage Vlimit and the like are generated in internal circuits in many cases. Therefore, the reference voltages are affected significantly by variations in parameters in IC production processes or changes in the ambient temperature during operation.
In the comparator circuit 1 shown in FIG. 8, even if a change occur within the variation range of parameters and within the operation temperature range, it is necessary to satisfy the relationship of the following equation (1).
Vlimit greater than Vthxe2x80x83xe2x80x83(1)
When Vlimit becomes smaller than Vth because of a change in the ambient temperature or the like, the input voltage Vsig exceeds the limit voltage Vlimit before reaching the hysteresis threshold voltage Vth, and is limited by the limiting circuit 3. Therefore, the hysteresis comparator 8 continues to be off, and does not transit to be on, and the output pulse does not normally operate.
In the conventional technique disclosed in JP-A 10-187862, operation in a comparatively wide temperature range is made possible by matching the temperature dependence of the reference voltage corresponding to the limit voltage Vlimit and that of the reference voltage corresponding to the threshold voltage Vth. However, this is not sufficient for 2.4 V or 1.8 V, which has been required for promotion of low voltages in recent years as the lowest operation voltage.
Furthermore, when the limit voltage Vlimit is set sufficiently high or the limiting circuit 3 is not provided, then the ON period of output becomes long and the pulse width becomes wider than necessary. With promotion of low voltages, the hysteresis voltage width Vhis cannot be ensured sufficiently, which causes malfunction such as chattering.
It is an object of the present invention to provide a comparator circuit in which the pulse width of an output can be controlled easily by operating a limiting circuit appropriately even if the power voltage is low, and an infrared signal receiving apparatus.
The present invention is directed to a comparator circuit comprising a limiting circuit for limiting an input voltage; a hysteresis comparator for performing discrimination of the input voltage, an output being changed so as to be on or off based on a result of the discrimination; and a voltage setting circuit for setting such that a limit voltage serving as a reference for limiting the input voltage by the limiting circuit changes in accordance with an operation of the hysteresis comparator circuit.
According to the invention, the comparator circuit includes a voltage setting circuit to limit an input voltage with a limiting circuit, perform discrimination with a hysteresis comparator and change an output so as to be on or off based on the discrimination result. The voltage setting circuit sets the limit voltage serving as a reference for limiting the input voltage so as to change in accordance with an operation of the hysteresis comparator circuit. Since the limit voltage changes in accordance with the operation of the hysteresis, comparator circuit, the relationship with the discrimination level of the hysteresis comparator circuit can be adjusted in accordance with the operation, so that the hysteresis comparator circuit can be operated appropriately even in a low voltage operation. The pulse width of an output can be controlled by adjusting the limit voltage of the limiting circuit.
According to the invention, the limit voltage of the limiting circuit changes in accordance with the operation of the hysteresis comparator circuit, the hysteresis comparator circuit can be operated appropriately even in a low voltage operation. The pulse width of an output can be controlled by adjusting the limit voltage of the limiting circuit.
In the invention, it is preferable that the voltage setting circuit sets the limit voltage to be high when the output is off, and to be low when the output is on.
According to the invention, since the voltage setting circuit sets the limit voltage to be high when the output of the hysteresis comparator circuit is off and the discrimination level is higher by the width of a hysteresis voltage, a normal operation of the hysteresis comparator circuit can be ensured. When the output of the hysteresis comparator circuit is on and the discrimination level is lower by the width of a hysteresis voltage, the voltage setting circuit sets the limit voltage to be low, so that the output pulse width can be controlled appropriately so as not to be too wide.
According to the invention, since the limit voltage is set high when the hysteresis comparator circuit is off, a normal operation of the hysteresis comparator circuit can be ensured. Since the limit voltage is set low when the hysteresis comparator circuit is on, the output pulse width can be controlled appropriately so as not to be too wide.
In the present invention, it is preferable that the voltage setting circuit sets the limit voltage in response to a discrimination level that changes in accordance with hysteresis in the hysteresis comparator circuit.
According to the invention, the limit voltage is changed in response to the discrimination level of the hysteresis comparator circuit, so that the limit voltage also can be provided easily with the hysteresis characteristics similar to those of the discrimination level.
According to the invention, the limit voltage of the limiting circuit can be provided easily with the hysteresis characteristics similar to those of the discrimination level of the hysteresis comparator circuit.
In the present invention, it is preferable that the limiting circuit includes:
an upper limiter for limiting the input voltage in a range of voltages that are higher than the discrimination level; and
a lower limiter for limiting the input voltage in a range of voltages that are lower than the discrimination level.
According to the invention, since the limiting circuit include an upper limiter for limiting the input voltage in a range of voltages that are higher than the discrimination level, and a lower limiter for limiting the input voltage in a range of voltages that are lower than the discrimination level, the output pulse width can be controlled more appropriately.
According to the invention, since the limiting circuit includes the upper limiter and the lower limiter, the output pulse width can be controlled more appropriately.
In the invention, it is preferable that the voltage setting circuit includes a buffer for inputting an operation of the hysteresis comparator circuit.
According to the invention, the operation of the hysteresis comparator circuit can be reflected on the limit voltage via the buffer, so that the operation of the limiting circuit does not affect the hysteresis comparator circuit.
Furthermore, according to the present invention, the buffer makes it possible that the operation of the limiting circuit does not affect the hysteresis comparator circuit.
In the invention, it is preferable that the voltage setting circuit sets the limit voltage such that the limit voltage is provided with temperature change characteristics of the hysteresis comparator circuit to match the voltage setting circuit to the temperature change characteristics.
According to the invention, even if the operation characteristics of the hysteresis comparator circuit are affected by the temperature and are varied, the temperature range in which the hysteresis comparator circuit operates normally can be widened because the limit voltage of the limiting circuit is similarly affected and varied by the temperature.
According to the invention, the temperature range in which the hysteresis comparator circuit operates normally can be widened.
Furthermore, the invention is directed to an infrared signal receiving apparatus comprising:
the comparator circuit as described above;
an infrared receiving circuit for receiving an infrared signal and generating an electric signal corresponding to the infrared signal;
a signal extracting circuit for selectively extracting a signal suited for a predetermined condition from electrical signals from the infrared receiving circuit; and
an integrating circuit for performing integration processing with respect to a signal extracted by signal extracting means and supplying an integrated output as an input voltage to the comparator circuit.
According to the invention, the infrared signal receiving apparatus includes an infrared receiving circuit, a signal extracting circuit and an integrating circuit as well as the comparator circuit as described above. Even in a low voltage operation, using the comparator circuit in which the pulse width of an output can be adjusted as appropriate, it can be determined appropriately whether or not a carrier is included in an output obtained after an integrating circuit integrates a signal extracted selectively as being suited for a predetermined condition by the signal extracting circuit from electrical signals generated after the infrared receiving circuit receives an infrared signal.
Furthermore, according to the invention, even in a low voltage operation, it can be determined appropriately whether or not a carrier is included in an infrared signal with the comparator circuit in which the pulse width of an output can be adjusted as appropriate.